This invention relates to a hydraulic power steering system having a double-acting servo motor which, with respect to the drive, is connected to mechanical steering elements of steering vehicle wheels; a controllable servo valve arrangement for the controllable connection of the servo motor with a hydraulic pressure source or a relatively pressureless reservoir; an electric or electronic desired-value generator which is operated by a manual steering device, particularly a manual steering wheel, and which is connected with an input of an electronic control path which, on the output side, is connected with the servo valve arrangement and which, by way of another input, is connected with an actual-value generator reflecting the steering position of the steering vehicle wheels and can be constantly tested automatically with respect to a faultless function; and a hydrostatic driving connection arranged between the manual steering device and the steering elements which is normally inoperative and which automatically becomes operative in the case of faults of the control path.
A power steering system of this type, without any mechanical forced coupling between the manual steering device and the steering vehicle wheels, is the object of German Patent document DE 38 37 395 A1 (corresponding to U.S. Pat. No. 4,955,445). In the case of this known system, the manual steering wheel actuates during the normal operation only an electric desired-value generator which is part of a control system for controlling the servo valve. This servo valve is constructed such that, in its center position, it hydraulically blocks the servo motor; that is, the motor connections are blocked. In addition, the manual steering wheel can be connected with the servo motor by way of a pump which is mechanically connected with the servomotor. The pump connections are hydraulically arranged between the connections of the servo motor and the assigned connections of the servo valve. As soon as the failing function of the control system controlling the servo valve is determined, the servo valve is moved into its center position and the pump is hydraulically coupled with the servo motor with respect to the drive. It is true that in the case of this power steering system, faults in the electronic control system of the servo valve can basically be controlled because then the hydraulic forced coupling between the servo motor and the pump driven by the manual steering wheel becomes operative. However, there is no safety should the servo valve block in a position deviating from the center position. In such a case, a hydraulic forced coupling between the servo motor and the pump would not be possible.
In the case of a steering system without a mechanical forced coupling between the steering vehicle wheels and the manual steering device known from International Patent document WO 88/09281 (corresponding to U.S. Pat. No. 4,736,811), a servo valve, which is used for controlling a servo motor provided for the steering operation of the steering vehicle wheels, can, on the one hand, be controlled electrically and, on the other hand, can be controlled hydraulically. In the normal operation, an electric desired-value generator is actuated by the manual steering wheel and is assigned to a control path for controlling the servo valve. Should a fault occur in this control path, a hydraulic circuit switches into operation which now permits a hydraulic control of the servo valve by way of the manual steering wheel.
From German Patent document DE-OS 22 33 167 (corresponding to U.S. Pat. No. 3,832,849), a hydrostatic steering system is known in the case of which the manual steering wheel and the steering vehicle wheels are hydraulically connected with one another with respect to the drive for their steering operation. As a function of the hydraulic pressures in the above-mentioned driving connection, a servo valve is controlled. The servo valve connects a servo motor connected with steering gears of the steering vehicle wheels. The servo valve is controlled in such a manner with a pressure source or a relatively pressureless reservoir that a servo power is generated which assists the respective steering maneuver and therefore reduces the manual force-required at the manual steering wheel.
German Patent document DE 41 33726 A1 shows a hydrostatic steering system with two parallel hydraulic circuits, each of which are separately sufficient for implementing steering maneuvers. One hydraulic circuit has a valve arrangement for controlling a first hydraulic servo motor of the steering system, which valve arrangement is mechanically operated by the manual steering wheel. The second hydraulic circuit has an electrically operated valve arrangement for controlling a second hydraulic servo motor for the steering vehicle wheels. In this case, the electric control elements of the valve arrangement are controlled as a function of a desired-value generator actuated by the manual steering wheel. This results in a so-called asymmetrical redundancy.
From German Patent document DE-AS 21 23 933, a hydrostatic steering system with a hydraulic servo assistance is known which is constructed such that a reaction power can be sensed at the manual steering wheel which is a function of the pressure conditions on the hydraulic servo motor for the steering vehicle wheels.
In German Patent document DE 37 29 898 C2, a hydraulic actuating drive for an automatic auxiliary rear wheel steering system of a motor vehicle is described. The hydraulic system is designed in such fashion that the auxiliary steering system is automatically blocked when the energy supplied to actuating assemblies of hydraulic control elements is switched off.
Japanese Patent document JP 62-15167 A shows a system which is similar to that of the initially discussed German Patent document DE 38 37 395 A1. During the normal operation, a manual steering wheel actuates an electric desired-value generator which is part of a control path for controlling a valve arrangement which, in turn, controls a hydraulic actuating drive of the steering vehicle wheels. In the case of a faulty operation of the control path, the mentioned actuating drive is hydraulically coupled with a hydraulic generator assembly which, with respect to the drive, is mechanically connected with the manual steering wheel.
Currently, as a rule, hydraulic power steering systems are installed in motor vehicles in the case of which a manual steering wheel is constantly connected mechanically with respect to the drive with the steering vehicle wheels for the steering operation. In this case, the servo valve is typically controlled by parts of the mechanical driving connection, particularly a steering column, between the manual steering wheel and the steering elements. For this purpose, parts of this driving connection can be adjusted relative to one another as a function of the transmitted forces or moments. The respective adjusting movement is transmitted to control parts of the servo valves.
In the case of such power steering systems, a so-called reaction control is also known which has the purpose of changing the manual forces to be applied at the manual steering wheel for its adjustment as a function of servo forces by means of which the servo motor assists the respective steering maneuver.
Such reaction control systems may operate hydraulically, in which case control parts of the servo valve are provided with separate effective areas which can be acted upon by hydraulic pressure which can be changed analogously to the pressure conditions at the servo motor in order to oppose an adjustment of the control parts by means of a more or less large resistance. In this case, the mentioned effective surfaces, in a regular manner, form parts of displacement chambers whose volume changes when the control parts are adjusted.
There is therefore needed a new power steering system, while using as many tested, reliable system parts as possible, which ensures a particularly high safety despite the elimination of a mechanical through-drive between the manual steering device and the steering vehicle wheels.
In the case of a hydraulic steering system of the above-mentioned type, these needs are achieved in that the hydrostatic driving connection is arranged independently of, or in parallel to, the hydraulic system of the system controlled by the servo valve and, in the case of the faulty operation of the control path, the servo motor can be switched into a free-running condition by the servo valve and/or by a shut-off valve which is closed only in the case of a faultless operation of the control path.
The invention is based on the general idea of arranging the hydrostatic driving connection separately from the hydraulic system of the servo system, and for providing multiple possibilities for switching the servo motor into a free-running condition. As a result, the highest safety can be provided in the case of faulty operations of the control path as well as in the case of defects of the hydraulic system of the servo system.
During the normal operation, the control path operates such that the manual steering wheel and the steering vehicle wheels are operatively connected electronically by means of wire; that is, the system according to the invention can be characterized as "steer by wire". In an emergency, a hydraulic forced coupling becomes operative between the manual steering device and the vehicle wheels.
According to a preferred embodiment of the invention, the hydrostatic driving connection communicates by way of control lines with hydraulic control input of the servo valve so that, when the hydrostatic driving connection is switched operative, the servo valve is actuated hydraulically and the servo motor is correspondingly controlled for assisting the steering maneuver.
This can be implemented in a simple manner in that the initially mentioned servo valves with a reaction control are used and the effective surfaces of the reaction control are acted upon by pressures in the hydrostatic driving connection.